Department of Earth System Science

Low latitude paleoclimate records from speleothem δ ¹⁸O measurements are often considered to reflect variations in precipitation amounts. Here we test this interpretation with a water isotope enabled atmospheric general circulation model, comparing modern and Last Glacial Maximum (LGM) controls on the δ ¹⁸O_p in Brazil. We argue that the δ ¹⁸O_p is determined by the contributions from local evaporation (high δ ¹⁸O) versus water vapor convergence to the region (low δ ¹⁸O). Our analysis indicates that the amount effect, commonly used to infer precipitation amount from δ ¹⁸O_p in low latitude regions, works only where the isotopic composition of incoming vapor stays relatively constant such as in coastal regions of the subtropics. If the isotopic composition of incoming vapor has changed as a result of the variations in the upstream rainout, δ ¹⁸O_p cannot be used to estimate local precipitation. Our analysis supports the increase of precipitation over northeastern Brazil region during the LGM.